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Properties of Dark Energy ?

  1. Aug 30, 2003 #1
    Properties of "Dark Energy" ?

    Has their been any progress in discovering what properties this Dark Energy might have?
  2. jcsd
  3. Aug 30, 2003 #2


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    Re: Properties of "Dark Energy" ?

    Depends on what you think constitutes progress. For starters, the main feature of dark energy
    cosmologists want to get a grip on is the "w" parameter
    and according to Michael Turner's review they have made some progress in narrowing down the range of uncertainty about "w"

    see for example

    section 3.5 "The Universe: The Lab for Studying Dark Energy"

    "Present cosmological observations prefer w = -1, with a 95% confidence limit w < -0.6"
    "A high-quality sample of 2000 supernovae distributed from z = 0.2 to z = 1.7 could measure w to within a precision &sigma;w = 0.05..."

    section 3.4 "Parametrizing Dark Energy: For Now, It's w"

    Here's my comment on Turner's survey article: OUT OF HUNDREDS OF QUESTIONS about dark energy that cosmologists could be asking the foremost question they want answered is what is the value of w, the parameter describing dark energy that most affects their models and the predictions about the universe---w is the parameter that determines the negative pressure that accelerates expansion (otherwise the expansion of space would be slowing down).

    TURNER SAYS THEY'VE MADE SOME PROGRESS and can say based on observations of supernovae so far that with 95% confidence w is less than -0.6

    they think it is -1.0

    but there is a confidence gap between -0.6 and -1.0

    to narrow it down to a sigma (stand. statistical deviation) of 0.05, he says, they would need to observe 2000 supernovae at moderate to high redshifts. That means getting observation time on expensive instruments like the Hubble space telescope and
    whatever will be launched as its successor.


    We have talked about the dark energy parameter w some already at PF and if you want we can discuss it some more. It is not hard to understand w, what it tells about dark energy and why they think it is -1.
    There are a whole bunch other questions people can speculate about, but the key one for connecting observations and predictions is this w parameter. Ask more if you want to know more.

    edit: your question made me look for more recent articles by Turner and others---there is an august 2003 survey article by him and Wendy Freedman that says they now can say w < - 0.8 with 95 percent confidence-----some improvement over the w < -0.6 I quoted from last year. The parameter w is called the "equation of state" for dark energy and is the ratio of its pressure to its
    Last edited: Aug 30, 2003
  4. Sep 8, 2003 #3
    Is it possible that the writer who suggests observations on 2000 supernovas is pointing to the fact that the accelerated expansion data came from a handful of supernovas and it's still ok to imagine the universe of last century?
    Please consider looking at my post, CONTINUOUS CREATION in Theoretical Forum. I haven't learned to use symbols so the math is unruly---and unnecessary; it only shows that accepting the postulates precludes a fully developed black-hole. Note it is testable as it explains why galaxy arms spin fast.
  5. Sep 9, 2003 #4
    does dark energy=vaccum energy?
    if it's true then why it has two names?
  6. Sep 10, 2003 #5


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    Perhaps. That is, vaccum energy is one candidate to explain dark enrgy, but we do not yet know whether it is the correct explaination. The biggest problem so far, as I understand it, is that all attempts to measure vaccum energy have come up with energy densities billions or trillions of times greater than the appearent amount of dark energy.
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